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doc: 1-byte varlena headers can be used for user PLAIN storage

This also updates some C comments.Reported-by: suchithjn22@gmail.comDiscussion:https://postgr.es/m/167336599095.2667301.15497893107226841625@wrigleys.postgresql.orgAuthor: Laurenz Albe (doc patch)Backpatch-through: 11

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Lines changed: 1 addition & 3 deletions

Original file line number	Diff line number	Diff line change
`@@ -444,9 +444,7 @@ for storing <acronym>TOAST</acronym>-able columns on disk:`
`444`	`444`	`<listitem>`
`445`	`445`	`<para>`
`446`	`446`	`<literal>PLAIN</literal> prevents either compression or`
`447`		`- out-of-line storage; furthermore it disables use of single-byte headers`
`448`		`- for varlena types.`
`449`		`- This is the only possible strategy for`
	`447`	`+ out-of-line storage. This is the only possible strategy for`
`450`	`448`	`columns of non-<acronym>TOAST</acronym>-able data types.`
`451`	`449`	`</para>`
`452`	`450`	`</listitem>`

Lines changed: 10 additions & 1 deletion

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`@@ -69,7 +69,16 @@`
`69`	`69`	`#include"utils/memutils.h"`
`70`	`70`
`71`	`71`
`72`		`-/* Does att's datatype allow packing into the 1-byte-header varlena format? */`
	`72`	`+/*`
	`73`	`+ * Does att's datatype allow packing into the 1-byte-header varlena format?`
	`74`	`+ * While functions that use TupleDescAttr() and assign attstorage =`
	`75`	`+ * TYPSTORAGE_PLAIN cannot use packed varlena headers, functions that call`
	`76`	`+ * TupleDescInitEntry() use typeForm->typstorage (TYPSTORAGE_EXTENDED) and`
	`77`	`+ * can use packed varlena headers, e.g.:`
	`78`	`+ * CREATE TABLE test(a VARCHAR(10000) STORAGE PLAIN);`
	`79`	`+ * INSERT INTO test VALUES (repeat('A',10));`
	`80`	`+ * This can be verified with pageinspect.`
	`81`	`+ */`
`73`	`82`	`#defineATT_IS_PACKABLE(att) \`
`74`	`83`	`((att)->attlen == -1 && (att)->attstorage != 'p')`
`75`	`84`	`/* Use this if it's already known varlena */`

Lines changed: 2 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -2383,7 +2383,8 @@ range_contains_elem_internal(TypeCacheEntry typcache, RangeType r, Datum val)`
`2383`	`2383`	`* values into a range object. They are modeled after heaptuple.c's`
`2384`	`2384`	`* heap_compute_data_size() and heap_fill_tuple(), but we need not handle`
`2385`	`2385`	`* null values here. TYPE_IS_PACKABLE must test the same conditions as`
`2386`		`- * heaptuple.c's ATT_IS_PACKABLE macro.`
	`2386`	`+ * heaptuple.c's ATT_IS_PACKABLE macro. See the comments thare for more`
	`2387`	`+ * details.`
`2387`	`2388`	`*/`
`2388`	`2389`
`2389`	`2390`	`/* Does datatype allow packing into the 1-byte-header varlena format? */`

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